In a 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see non patent document 1).
In the D2D communication, a plurality of adjacent user terminals perform communication in a frequency band assigned to the mobile communication system without a core network. It is noted that the D2D communication is also called Proximity Service communication.